As is well known, various high performance electronic devices must detect signals which are almost completely masked or hidden by signal clutter or interference. The problem, however, involves more than the ability to ascertain a target because, large clutter may also drive the receiver into non-linear operation. When this happens, distortion in the received signal may either appear as false alarms or mask small targets. For example, in the case of Doppler processing radars, the radar must be able to reliably detect and track low radar cross-section targets in the presence of large clutter or severe electronic counter measures (jamming). To do this, these radar systems must process extremely large signals with sufficient sensitivity to search through the clutter or jamming for a weak return from the low radar cross-section target. As a consequence, it is desirable that the receiver add little, if any, noise or distortion to the signal being processed. Further, it is desirable that the receiver maintain linearity over an extremely wide dynamic range of signal levels.
One method for removing system distortion from the received input signal, to thereby extend the intercept point of an amplifier without increasing the output saturation power, is called "feedforward intermodulation distortion cancellation." In essence, this method requires coupling the amplified output signal, which includes distortion added by the amplifier, with the input signal to the amplifier in order to cancel the input signal component from the amplified output signal. The result is a distortion signal. This distortion signal is then amplified and subtracted from the amplified output signal to generate an output signal from which amplifier distortion has been canceled and which is a desired amplification of the input signal.
Conventional circuitry for performing "feedforward intermodulation distortion cancellation" requires two cancellation loops which are formed using a power amplifier, an error amplifier, and four directional couplers. For this conventional circuitry, one directional coupler is used to direct a portion of the input signal to be amplified to the power amplifier. Two directional couplers interconnect the output from the power amplifier with the error amplifier so that the input to the error amplifier will include both a portion of the input signal and a portion of the output from the power amplifier. This connection results in isolating the distortion added by the power amplifier. This distortion is then amplified by the error amplifier. A fourth directional coupler is used to combine the distortion signal output from the error amplifier with the output from the power amplifier to cancel the distortion component from the output of the power amplifier. The result is an amplified signal from which the distortion that was added by the power amplifier has been canceled.
The conventional circuitry, however, suffers from at least two drawbacks. First, it incorporates four directional couplers. These components are bulky and thus, take up space which might otherwise be used to advantage. Second, for this conventional circuitry to work properly, both cancellation loops must be very precisely controlled. It happens that any phase or gain shift in the cancellation loops, such as will occur over time and with variations in temperature, reduces the distortion cancellation abilities of the circuitry. Thus, an implementation which maintains close tracking between critical device parameters is desired. The present invention recognizes that a monolithic microwave integrated circuit (MMIC) has such tracking properties and, that a monolithic circuit can be used with fewer directional couplers to accomplish results similar to those obtained with "feedforward intermodulation distortion cancellation" using conventional circuitry.
In light of the above it is an object of the present invention to provide a MMIC amplifier having an extended dynamic range which minimizes amplifier distortion in the amplified signal. It is another object of the present invention to provide an amplifier which maintains linear operation throughout an extended dynamic range. Yet another object of the present invention is to provide an amplifier which closely tracks the critical device parameters. Still another object of the present invention is to provide an amplifier which requires a minimal number of directional couplers to accomplish the feedforward function of removing undesirable distortion from the amplified signal. Another object of the present invention is to provide a MMIC amplifier having an extended dynamic range which is simple to use, relatively easy to manufacture and comparatively cost effective.